Semiconductor multilayer mirrors are used in a variety of devices, e.g., in surface-transmitting lasers. Typically these mirrors consist of alternating layers of a first and a second semiconductor material, with each layer having optical thickness .lambda./4. The "optical thickness" is the actual thickness multiplied by the refractive index of the material at the wavelength .lambda.. The theory of multilayer optical elements is well known. See, for instance, "Principles of Optics", M. Born and E. Wolf, 5th edition, Pergamon Pres 1975, pages 51-70, especially pages 66-70, which deal with periodically stratified media.
It is well known that essentially defect-free epitaxial growth of a (lattice mismatched) second semiconductor on a first semiconductor is in general only possible as long as the thickness of the second semiconductor layer does not exceed the so-called "critical thickness" L.sub.c. Such strained material is referred to as "pseudomorphic". See, for instance, U.S. Pat. No. 4,861,393. The thickness L.sub.c depends, inter alia, on the lattice constant difference (the lattice "mismatch") between the first and the second semiconductor material, as exemplified by FIG. 1 for the case of Si/Ge.sub.x Si.sub.1-x. As shown by FIG. 1, if x+0.1 then L.sub.c is about 5 .mu.m. The curve of FIG. 1 is based on measurements for x.gtoreq.0.16 and is extrapolated for x&lt;0.16.
The existence of the relationship between lattice mismatch and L.sub.c implies that a stack of Si/Ge.sub.x Si.sub.1-x layers, each of optical thickness .lambda./4, can be grown essentially defect (i.e., dislocation) free on Si only if the total thickness of the stack is less than L.sub.c appropriate for the average composition of the stack. For instance, for .lambda.=1.3 .mu.m, the appropriate actual layer thickness of 92.8 nm for Si and 89.0 nm for Ge.sub.0.25 Si.sub.0.75. The average composition of a .lambda./4 stack (for 1.3 .mu.m radiation) of Si/Ge.sub.0.25 Si.sub.0.75 thus is about Ge.sub.0.122 Si.sub.0.878, i.e., &lt;x&gt;=0.122, and L.sub.c for such a stack is about 2.25 .mu.m. A pseudomorphic .lambda./4 stack of Si/Gi.sub.0.25 Si.sub.0.75 on Si thus can contain at least 12 layer pairs. Thicker stacks will contain dislocations that provide stress relief.
Those skilled in the art will readily appreciate that the presence of dislocations in opto-electronic and other semiconductor device is highly undesirable. On the other hand, as exemplified above, pseudomorphic conventional multilayer mirrors typically would have relatively few layers and therefore have low reflectance, resulting typically in devices of relatively low efficiency.
In view of the importance of, e.g., efficient opto-electronic devices, would be desirable to have available a pseudomorphic multilayer second/first semiconductor layer stack on a first semiconductor substrate that contains more layer pairs than is provided by the prior art. This application discloses such a combination, and articles comprising the combination.